1. Field of the Invention
This invention pertains to hydraulic control systems, and is directed more specifically to a control system for automatically adjusting the displacement of a variable displacement hydraulic pump or pumps or for limiting the input torque thereof at a predetermined value.
2. Description of the Prior Art
A variety of hydraulic control systems have been suggested and used for automatically adjusting the displacement of a variable displacement pump or a plurality of such pumps driven by a common prime mover such as an internal combustion engine. Included in such control systems are input torque limiters whereby pump displacement is adjusted in accordance with pump output pressure so that the product of the pump displacement and the pump output pressure may be maintained at or close to a predetermined limit.
A well known example of the input torque limiters is of the cam type wherein a cam is actuated in accordance with pump displacement via a linkage system for correspondingly adjusting the force of a spring on a control valve which senses the pump output pressure. This cam type input torque limiter is objectionable in that the linkage system in use must be a highly complex and precision-made one. Moreover, for combined use of the torque limiter with a load sensing or cutoff control system, a still more complex and precision-made linkage system is needed for stabilized operation. A further objection is that the input torque limit of the pump under control is determined by the spring modulus of the control valve and by the contours of the cam and, therefore, cannot be easily re-adjusted or altered. As a consequence, in the case where the variable displacement pump is driven together with some other pump by a common prime mover, it is impossible to match prime mover output torque and pump input torque.
Another known input torque limiter is of the multiple-stage spring type, wherein the pump displacement is decreased linearly with an increase in the pump output pressure. In order to closely approximate the product of pump displacement and pump output pressure to a predetermined limit, the linearity characteristic is modified in accordance with the pump output pressure by the multiple-stage spring means. This second known example is unsatisfactory in control accuracy, however, and is also not readily adaptable for use with a load sensing or cutoff control system. Further, the input torque limit of the pump is determined by the modulus and the type of the spring means in use, thus preventing easy re-adjustment.
There has also been known a pump displacement control system which is responsive to prime mover speed, such that the pump displacement is decreased upon decrease in the pump speed. Although the pump input torque is not strictly maintained at a constant value, the speed responsive control system permits proper torque matching even in the case where the variable displacement pump is driven together with another pump by a common prime mover. An objection to this known control system, however, is that the pump speed varies with the pump output pressure. Especially in the use of a prime mover of low torque rise, the output power of the pump decreases with an increase in its output pressure. Moreover, the response of the system is comparatively poor because of the inertial forces of the prime mover.
These drawbacks result from unnecessarily great fluctuations of pump speed. In order to reduce such speed fluctuations, there has been suggested and used a system incorporating a summing valve as an auxiliary control mechanism, for limiting the total input torque of a plurality of variable displacement pumps. The summing valve helps to accomplish this objective only to such an extent, however, that unnecessary speed fluctuations cannot be eliminated altogether.